Fix clang warning "has virtual functions but non-virtual destructor".

src/IceFixups.h

Index: src/IceFixups.h
diff --git a/src/IceFixups.h b/src/IceFixups.h
index fb8a164eadc70306e32e27b317a21f99c9a34f11..1a29a18650ef67c600f6eace1cd07b5166b5de54 100644
--- a/src/IceFixups.h
+++ b/src/IceFixups.h
@@ -31,6 +31,7 @@ class AssemblerFixup {
 public:
   AssemblerFixup() = default;
   AssemblerFixup(const AssemblerFixup &) = default;
+  virtual ~AssemblerFixup() = default;
   intptr_t position() const {
     assert(position_was_set_);
     return position_;